I've found vnstat to be useful for monitoring bandwidth on Raspbmc (although it would work fine on Debian squeeze too)
Link here - http://www.debian-administration.org/articles/330
To install type - sudo apt-get install vnstat
Then to start it type - sudo vnstat -u -i eth0
I found that it wasn't updating the database automatically (i guess this may be a permission issue, beyond my beginner linux knowledge!) to get round this, when you want to view the stats type - sudo vnstat --update
Then just type - vnstat
You can also do vnstat --live to well erm see the activity live...
There are a variety of commands in the help page too.
Apologies if any of that is incorrect or useless, I'm a linux beginner and typing on my phone - typos highly likely!
I found this quite useful while live streaming with tvcatchup
Note: this will count all traffic to/from the pi regardless of wan/lan!